1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method using a discharger in which a corona discharging system is used, and forming an image based on image data on a paper by an electrophotographic system.
2. Description of the Related Art
An image forming apparatus using an electrophotographic process exposes a photosensitive member (image bearing body) charged by a charger based on image data to be processed, to form an electrostatic latent image on the photosensitive member. Moreover, the image forming apparatus supplies charged toner (developer) to the latent image by a developing unit, to form a toner image on the photosensitive member. The image forming apparatus transfers the toner image formed on the photosensitive member onto a paper by a transfer unit. The image forming apparatus then fuses the transferred toner image to the paper by a fuser, to form an image on the paper. Such an image forming apparatus may use, as a charger for charging the photosensitive member, a discharger in which the corona discharging system is used.
The charger of the corona discharging system includes a corona wire, formed by tungsten, stainless steel or the like, which is enclosed by a casing. The charger is arranged near the photosensitive member. When high voltage is applied to the corona wire, corona discharge occurs between the corona wire and the photosensitive member, the air around the corona wire is ionized, and ions are generated. The generated ions are moved to the surface of the photosensitive member to charge the surface of the photosensitive member.
It has been known that, in the charger of the corona discharging system, if dirt is adhered to the corona wire, the photosensitive member is not normally charged at the portion that contains the dirt, causing a charge failure at the photosensitive member. Such a charge failure results in significant deterioration in image quality because, for example, a solid black area may be formed on the printed image. It is thus necessary to sufficiently clean the corona wire in order to maintain good image quality.
For example, an image forming apparatus has been proposed, which includes a means for cleaning the corona wire in conjunction with opening and closing an access cover (see Japanese Patent Application Laid-Open No. 2006-251195). Another image forming apparatus has been proposed, which determines if there is dirt on a charger immediately after cleaning of the corona wire, and allows a printing operation to occur if it determines that there is no dirt (see Japanese Patent Application Laid-Open No. 2006-184826). By thus cleaning the corona wire on a regular basis and appropriately determining if there is dirt on the charger, the corona wire can be maintained in a clean condition, and it is possible to reduce the degradation of quality of the formed image.
The technique disclosed in Japanese Patent Application Laid-Open No. 2006-184826 has a configuration that detects charging current of the charger, determines if there is dirt on the charger based on the detected value of charging current, and provides a notification if there is dirt on the charger.
It is known, when the image forming apparatus forms an image on a paper, that two or more types of print speeds may be used for processing and that the print speed can be switched in accordance with, for example, the type of paper. Such an image forming apparatus requires a change in the standard used to determine the presence/absence of dirt on the charger, when the print speed is switched. This is because the rotation speed of the photosensitive member is also changed when the print speed is changed, and the voltage applied to the charger is further changed in order to obtain constant charging density.
The Japanese Patent Application Laid-Open No. 2006-184826, however, does not consider switching of the print speed. Therefore, in the image forming apparatus capable of processing with two or more types of print speeds, it is difficult to determine appropriately whether or not the corona wire needs to be cleaned depending on each print speed.